Frictional behavior of self-adaptive nanostructural Mo-Se-C coatings in different sliding conditions

Sliding properties of Mo-Se-C coatings with two different carbon content deposited by magnetron sputtering were investigated in different sliding environments (argon, nitrogen, dry and humid air). Both coatings had a structure that was identified as randomly oriented structures of MoSe2 embedded into amorphous carbon matrix. The worn surfaces, i.e. the wear tracks and the wear scars of the balls, were analyzed by optical microscopy, Raman spectroscopy and scanning electron microscopy. The material transferred to the ball steel surfaces was almost exclusively MoSe2, whereas the wear tracks on the coatings were more complex, with areas rich in MoSe2 and areas similar to that of as-deposited coatings. The friction was lowest in argon (0.012 at a load of 10 N) and highest in humid air, but still remarkably low; as best 0.05 at 10 N load; however, the exceptionally low wear rate was almost identical. Thus, we focused our detailed analysis on these two examples to understand the mechanisms responsible for the difference between the friction coefficients. SEM, EDX, XPS, Raman and TEM with EELS and EDX were applied to investigate the composition and structure of localized spots of interest on the tested surfaces. In both cases, we observed well-ordered MoSe2 tribofilms with negligible amount of oxides. Carbon was not present in the sliding interfaces, although large amount of carbon was found outside the contacts on both surfaces. Based on our investigations, we suggest the increase in friction of Mg-Se-C in humid air is primarily due to the increase in shear strength of the MoSe2 structure by the presence of water molecules in the sliding interface. (C) 2013 Elsevier B.V. All rights reserved.